joanna gets paid ?$50 an hour to DJ at Club Keynes on Friday and Saturday nights. The Club is closed all other nights. She also gets paid ?$15 an hour to work at the library. The library will let her work as many hours as she has available. Club Karl wants to hire Joanna to DJ on SaturdaySaturday nights. The marginal opportunity cost of hiring Joanna at Club Karl is ?$enter your response here an hour.